The use of inorganic corrosion inhibitors, e.g., metal oxides alone and/or in combination with organic inhibitors including organic phosphonic acids have been used in various aqueous systems. It has been found, in accordance with this invention, that certain amino phosphonic acids and its derivatives and particularly the amino methylene phosphonic acids having an increased number of methylene groups in combination with citric acid and/or the alkali metal salts thereof or combinations of a metal molybdate with citric acid and/or its alkali metal salts have improved corrosion inhibition. In addition, the compositions of this invention prevent the deposition of mineral scale normally encountered in aqueous systems.
In general, corrosion is defined as a destructive attack on metal involving an electrochemical or chemical reaction of the metal with its environment. More specifically, an electrochemical attack on a metal surface is the wearing away and undercutting of the metal, which is accelerated after the protective coating, e.g., the oxide film is removed by the corrosive medium. Other types of corrosion include cavitation and erosion wherein addition to an electrochemical reaction the condition of the aqueous systems are such that the continuous flow causes cavities where high pressure areas develop causing pressure and shock resulting in a pitted metal surface. This type of corrosion generally is found in water pumps, propellers, turbine plates, etc. Further, erosion of the metal surface will occur if the medium contains suspended solids which impinge the surface of the metal as the fluid is transported through the pipes thereby removing any protective film and exposing the metal to corrosion.
Presently, many corrosion inhibiting compositions are being used at low levels in an attempt to control corrosion. Often they contain in addition an agent for control of mineral scale formation which has a tendency to increase the rate of corrosion, and therefore stronger corrosion inhibitors at higher concentrations are used in order to obtain satisfactory results. Moreover, the use of some of these inhibitors such as the chromates at higher concentrations is unsatisfactory because of the environmental restrictions. It has been found by utilizing an amino methylene phosphonic acid and particularly an amino methylene phosphonic acid with an increased number of --CH.sub.2 -- groups, that a lower concentration of inhibitors can be used in combination therewith and in most instances even a weaker inhibitor will provide good results. Thus, the novel compositions of this invention eliminate the need for using inhibitors, e.g., toxic materials at the higher concentrations and provides a corrosion and scale inhibitor which is effective in different aqueous systems.